A typical method of making heat exchangers for a variety of gas and oil fired industrial or residential products is to bend a metal tube into a serpentine shape thereby providing multiple passes. Gases heated by a burner at one end of the heat exchanger travel through the tube interior and exit the other end of the heat exchanger. While the hot flue gases are within the tube, heat is conducted through the metal walls of the tube and transferred to the air or other fluid media surrounding the tube thereby raising its temperature. In order to achieve efficient heat transfer from the tubes, it is usually necessary to alter the flow of gases by reducing their velocity and/or promoting turbulence, mixing and improved contact with the tube surface. A typical method for achieving this is by placing a separate restrictive turbulating baffle inside the tube. These baffles are typically metal or ceramic. One problem associated with baffles in tubes is noise caused by expansion or contraction of baffles or vibrations generated by the mechanical coupling to components such as blowers or fans. Another difficulty related to the use of baffles is that the heat exchanger tube cannot be bent with a baffle already inserted so that baffles must be inserted after bending, limiting the typical location of baffles to straight sections of the heat exchanger tube which are accessible after bending. In addition, the use of separate baffles increases the cost and difficulty of assembling the heat exchanger.
A known alternative to baffles is the technique of selectively deforming the tube to change its cross section. Such deformation causes a restriction to the gas flow due to the change in cross section, achieving the effect of baffles. For example a known method is to flatten sections of the tube to achieve the desired restriction. A problem with the use of flattened sections is that this technique extends the cross section of the tube beyond that of the tube without deformations, creating low spots in horizontal sections. Additionally, the flattened sections prevent the tube from passing through a hole of approximately the tube outside diameter as required for assembly in some applications.
While deformation of the heat exchanger tube can replace the use of baffles in some applications, the deformation technique has had less than satisfactory results when applied in commercial and light commercial heating and air conditioning units. The design of most heating and air conditioning units is such that the heat exchanger is located downstream of the evaporator section for cooling. Therefore, during use for air conditioning the cool air passing over the heat exchanger lowers the tube temperature below the dew point of air inside the tube, resulting in condensation inside the tube. Current configurations of tube deformation experience problems in draining this condensation from the tube due to low spots in the horizontal sections of the tube. The low spots, which are caused by restricting deformations prevent the flow of liquid, allowing condensate to puddle and increase the likelihood of corroding the tube. For this reason baffles are often used in heating and air conditioning unit heat exchangers to avoid premature failure due to corrosion.